Recently, conversion of natural energies such as solar energy, terrestrial heat, wind power and tidal energy, or biological energies such as that of biomass is extensively studied because of the rising cost of petroleum energy. Especially, conversion of the wind energy into electricity or rotation energy have been widely used since the equipment for such conversion is relatively low-cost and highly durable.
Wind, however, extensively varies in velocity and easily shifts its direction dependent upon season, location and time of the day. As in the case of typhoon or gust, there is a fear for a sudden incidence of a powerful wind which is extremely higher in energy than those usually observed.
Accordingly, besides difficulties to obtain a energy at constant level with such variable wind, it is important that windmill should be protected from damage or destruction by typhoon or gust.
As to the means to protect windmill from harms by high wind, various attempts have been proposed: For example, Japan Patent Kokai No. 143,369/81 discloses an attempt to prevent the destruction of propeller-type windmill by high wind placed on the top of a tower by descending the windmill and turning up its nacelle. Japan Patent Kokai No. 41,467/81 discloses an attempt to prevent the destruction of propeller-type windmill by changing the pitch of its fliers.
The former attempt, however, requires a mechanical descending operation and night watchman, as well as having a disadvantage that the tower per se may not tolerate high wind.
In the latter attempt, although the fliers of the windmill may tolerate a relatively low wind, it is doubtful whether the whole windmill structure and strut tolerate a higher wind.